thunderstorm outflow boundary

Bryan Bjorkman

I hear talk of "outflow boundaries" from earlier storms, exactly what is this and how does it influence new development?
I don't know why I'm suddenly posting a bunch in here after going for months without saying anything, but oh well. :)

Think of a thunderstorm as a giant, perfect machine. It processes it's fuel (warm, moist air) gathered at the surface, raises it high inside where it condenses and forms rain/hail, etc., and then disperses what it has made in the form of downdraft. As the air rises and condenses, it also cools ... so the downdraft portion of the storm is full of rain-cooled air. Then as balance in the storm eventually is lost, you'll often hear chasers saying things like 'the storm gust out' or 'became outflow dominant' ... at this point the perfect machine becomes lopsided in favor of outflow, which is usually the beginning of the end for the cell. As that cool air hits the surface of the ground, it goes from moving horizontally to vertically. It will usually take on a somewhat circular pattern moving outward from the base of the storm. This cool air sticks around for quite a while - hours - overnight - into the next day - and can travel for miles and miles along the surface of the ground. Since a boundary is just a place where two air masses meet, this outflow from the storm makes little boundaries everywhere it goes. Chasers become super interested in outflow boundaries as the season goes on and going into summer ... because it is along these boundaries that storms feed off the energy between cold air on one side and moist warm air on the other and get really jiggy, or more tornadic. Outflow boundaries sometimes can take a set up that would be mediocre any other time and turn it into a tornadic funfest!

As far as how it influences new thunderstorms ... it works the same way as any frontal boundary that triggers convection. Think of a cold front as a wedge of air moving against the surface of the ground (not like a wall as most people commonly think) ... on its leading edge it is more narrow and shallow ... this has the effect of wedging up the warm, moist air ahead of it as it moves. Voila ... you have a source of lift, one of the primary ingredients for thunderstorms ... just add an element of vertical shear and easy access to moisture and the show is on!
What methods are used for actually indentifying if an outflow boundary is present? I know that boundaries often show up as a light ring on the radar, but what does one look for on the satellite (for outflow boundaries in particular). Also, which storms and environmental conditions will produce a lingering and sharp boundary, and which ones will fade quickly? Is it merely a matter of precipitation being present in the area over the past 12-18 hours?
Skip - as you mentioned, they can often be seen as little rings on radar ... and their location can also be determined by looking at satellite imagery, though sometimes it is more subtle and you have to be observant ... pockets of fine cumulus, or accas formations occur along boundaries - so on satellite you will just see areas where areas of fine lines of clouds indicate the boundary. On satellite they just look like a bunch of little white blobs, basically.

Edit - posted the above before I saw John's links ... they demonstrate the idea nicely.

In addition to Mike and the stuff I posted, the boundry can affect weather 24 to 48 hours later (sometimes more from what I've read). Of course that much time having gone by, they don't show up mear as clearly, but following the SPC's discussion as well as the loca NWSFO discussions, they will generally point out the aafect an outflow boundry may have on the current weather. Another point is that there needs to have been (or currently) a weakening T-storm in the vicinity to produce the gravity wave or outflow. Not just any precip.

Next time you're arm chairing it, watch a heavy squall line. You can see the outflow 10 to 20 miles (often further) in front of the line. then take a look at the same line on a visible satelitte view. Take note of what you see and apply it to an isolated Supercell and see how they look both radar and sat. Take a look the next morning on the radar after a storm fell apart and the potential for storms that day. You will see where the boundry can enhance the storm potential locally on an otherwise marginal day.
The HPC also does a pretty good job of showing outflow boundaries on their current analysis page. See boundary descriptions #5:

and current surface analysis:

They don’t currently show any labeled as “Outflowâ€￾ (I thought maybe there would be a few after yesterday in the SE), but as noted above, as we get deeper into spring and summer they will be plotted on this page.
I suspect (although not positive) that these can be found by carefully looking at surface plots? No expert here, but maybe someone more learned could point out which fields in analysis might indicate maybe pockets of cooler moistned air showing up in the data near areas of previous convection which could not otherwise be explained?

Nice discussion!
You are correct. OFB can usually be seen in surface observations by sharp gradients in temperature, dewpoint, wind direction, etc. Outflow air is typically cooler and more moist than inflow air (not always, but typically). Therefore, where you find a signficant gradient of such that is not associated with other boundaries (i.e. not the dryline, warm front, etc) is where an OFB is located. OFBs are of different sizes (in areal extent), so there are times when the porous surface observation system cannot capture the OFB (in other words, the outflow air lies between surface observation stations)... Othertimes, as can be the case with decaying squall lines or other mesoscale systems, OFBs can be many miles in length, so it's easy to pick out on a surface map.
It is also important to keep in mind that the observation of outflow boundaries (OFBs) on surface maps is only as effective as the density of observations available. We are spoiled in Oklahoma and other states such as Iowa with the dense network of weather stations (often referred to as a "mesonet"). In addition, temporal resolution must be better than the 20 to 60 minutes provided by weather service observation sites to identify these features effectively.

Unfortunately, radar is the most successful observation tool we have for OFBs at this point in time. Nowcasting systems such as NCAR's Auto-nowcaster are now using algorithms to identify these OFBs (as seen on John's image above) on radar imagery and predict when and where they will intersect eachother, as this is often a prime area for the initiation of convection.
Excellent thread and great discussion. I can't add anymore to the excellent links everybody provided, but I'd like to add some of my personal observations regarding outlfow boundaries and supercells.

I like good, strong outflow boundaries that a stationary or very slow moving and a result of overnight or early morning extgensive convection (small to moderate MCS are excellent) that dissipates or moves away to allow good insolation on either side of the boundary. These types of boundaries stand up well against strong surface flow throughout the day.

These preferred boundaries are almost always able to be analyzed on surface charts with distinct temperature and/or dewpoint differences and especially wind vectors. They are almost synoptic in nature like a cool/warm front. Sometimes, the difference on a surface chart is subtle like a 5F temp or dewpoint difference and subtle windshift. Overall, my general rule of thumb is that if the boundary shows up distinctly on radar and/or satellite and persists into th early afternoon, it's a boundary to be taken seriously. If I can't find it on radar/satellite as well as careful surface analysis, then I don't give it a second look.

Sometimes though, outflow boundaries, even strong ones, get diffused by strong surface flow and end up dissipating entirely by initiation. I find this true alot during the early part of the season where you get these crazy 50-60 knot 850mb jets with related surface winds.

Sometimes, there are things that look like outflow boundaries like a gravity wave..especially one trapped under a strong cap. These can easily fool you in a pre-afternoon analysis, so check the surface plots carefully. These things can lead you astray and look impressive only to completely disappear by early afternoon.

Of course, anytime you can get a dryline, cool front, or another outflow boundary intersecting it, then perform the "Forbidden Sacred Dance of Chaser Merriment" 8)

That's my $0.02 worth anyway.